Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al-Substituted Ferrihydrite: Implications for Ferrihydrite Surface Structure and Reactivity

Abstract

Ferrihydrite is a nanocrystalline Fe (hydr)oxide and important sink for environmental contaminants. Although Fe (hydr)oxides are rarely pure in natural systems, little is known about the effects of structural impurities such as Al on the surface properties and reactivity of ferrihydrite. In this study, we characterized the adsorption mechanisms of chromate, selenate, and sulfate on Al-substituted ferrihydrite (0, 6, 12, 18, and 24 mol % Al) using in situ attenuated total reflection Fourier transform infrared spectroscopy. Spectral data sets recorded as a function of pH were processed using a multivariate curve resolution technique to identify which types of surface species form and to generate their concentration profiles as a function of pH and Al content. Results show a significant increase in relative fraction of outer-sphere complexes for all three oxyanions with increasing Al substitution. In addition, the effect of Al substitution is found to be mechanism-specific in the case of chromate, with bidentate complexes disproportionately suppressed over monodentate complexes at higher Al contents. Overall, our findings have important implications for the fate of chromate, selenate, and sulfate in subsurface environments and offer new insight into the surface reactivity of Al-ferrihydrite.

Adsorption of Quinolone Antibiotics to Goethite under Seawater Conditions: Application of a Surface Complexation Model

“…As typically observed for anion adsorption to iron oxides, sulfate adsorption decreased with increasing pH. [31][32][33][34] According to several vibrational spectroscopy studies, 32,35,36 both inner-sphere complexes and outer-sphere complexes are supposed to form between SO 2and the goethite surface. Therefore, a monodentate MB complex (eq.7) and a monodentate HB surface (eq.8) complex can be proposed to describe the adsorption data as following: 37 H + + ≡FeOH -0.5 + SO 4 2-≡FeO -0.5+Δz0 -SO 3 Δz1 + H 2 O ; log K≡FeOSO 3 (7) ⇌ H + + ≡FeOH -0.5 + SO 4 2-≡FeOH 2 -0.5+Δz0 … Δz1 O-SO 3 Δz2 ; log K≡FeOH 2 SO 4 (8) ⇌…”

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite

“…Some impurities such as phosphate are important nutrients, while other impurities may be unwanted compounds, toxic metals and metalloids (e.g., Cd, Pb, As). Their occurrence and speciation are strongly dependent on the stability of ferrihydrite under environmental conditions [20][21][22]. Although pure ferrihydrite is unstable and directly transforms to more stable phases such as goethite or hematite [2], many recent studies have proved that the admixtures present in ferrihydrite significantly inhibit its transformation [23,24].…”

Layered Double Hydroxide Sorbents for Removal of Selenium from Power Plant Wastewaters

“…For these reasons, alternative technologies such as sorption are receiving interest for further development [9]. Selenium is mostly found as the selenate oxoanion, SeO 2− 4 , in aerobic environments, which is difficult to remove via adsorption due to its tendency to bind via outer sphere mechanisms on common sorbents [13][14][15][16]. This, along with its chemical and physical similarities to sulfate, make selectivity of common sorbents to selenate a challenge.…”